1. Field of the Invention
This invention relates generally to transmission of signals, and, more particularly, to bi-directional strobe transmission with center-tapped termination.
2. Description of the Related Art
The demand for quicker and more powerful personal computers has led to many technological advances in the computer industry, including the development of more efficient and powerful processors. The future generation of high performance processors are projected to operate at much higher frequencies than today's processors. Processors operating at higher frequencies, however, can be susceptible to timing noise, such as inter symbol interference (ISI) noise, for example.
The ISI noise is undesirable because it is capable of disrupting the timing delays in a logic circuit, especially in logic circuits utilizing a source synchronous scheme. In a source synchronous scheme, strobe signals are transmitted along with the data.
A strobe is a signal used by a receiver to latch the data transmitted by a driver. Strobes can be implemented in logic circuits in a variety of ways. One implementation, for example, is to utilize two strobes (STB and STB#), where the receiver utilizes the two strobes to latch four packets of data that accompany the strobe signals. The strobes STB and STB# are inverses of each other. Typically, the first transition of STB (STB#) is low (high) to high (low), and each strobe transition (rising) is used by the receiver to identify and latch the correct packet of data. Thus, during the first transition of STB, the receiver will latch the first packet of the data, and during the first transition of STB#, the receiver will latch the next packet of the data, and so on. The transition completes when all four packets of the data have been latched by the receiver. After the data transfer is complete, the strobes are maintained at voltage levels substantially identical to initial voltage levels. Thus, the strobe STB is maintained low, while the strobe STB# is maintained high.
One well-known method for reducing the ISI noise during the transmission of signals is commonly referred to as center-tapped termination (CTT). To reduce the ISI noise, it is desirable to apply CTT to strobes as well. However, applying CTT to strobes in a bi-directional transmission scheme causes the strobes to drift to a metastable state when the strobes are not driven during a bus turnaround cycle. A bus turnaround cycle is the transition period during which a driver, which was initially driving the strobe signal, becomes a receiver, and the receiver, which was initially receiving data, becomes the driver. The driver initially driving the strobes is hereinafter referred to as the "current-driver," whereas the driver that subsequently drives the strobes is referred to as the "next-driver."
Because it is possible that neither the current-driver nor the next-driver is driving the strobes during the turnaround cycle, the strobes can drift to a metastable state because of CTT. Typically, it is the bus keepers that sustain the strobes at valid voltage levels during a bus turnaround cycle. However, because these bus keepers only supply small current-driving levels to sustain the strobes at valid voltage levels, the high current-driving levels of CTTs override the bus keepers, causing the strobes to drift to one-half of V.sub.cc (power supply). This metastable state causes the strobe state machine at the receiver to go to an incorrect state, thereby hampering the use of CTT on strobes. Thus, there is a need of a method and apparatus of implementing CTT on strobes in a bi-directional transmission scheme that would prevent the strobes from drifting to a metastable state.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.